36 Concrete in Australia Vol 39 No 4
FEATURE: RHEOLOGY
Optimising the fresh properties of concrete
by understanding rheology
James Mackechnie, South Island Plant Engineer, Allied Concrete, New Zealand
Project specifications have specific requirements to achieve intended hardened properties but often do not make suitable allowance
for fresh properties of concrete. Understanding the rheological properties of concrete allows better optimisation of materials that
influence workability. Characterising the yield shear stress and plastic viscosity of concrete provides a more scientific assessment
of fresh properties and complements the intuitive feel developed by experienced concrete technologists. Design of special concrete
mixes is currently done using a heuristic approach and product development is therefore fairly conservative. This paper presents
findings from rheological studies of concrete where the effects of materials, mix designs and specifications are compared, and the
advantage of this approach is shown. The influence of supplementary cementitious materials, aggregates, chemical admixtures
and fibres on concrete properties is discussed and implications for these factors on specifications are explained.
1.0 INTRODUCTION
Fresh properties of concrete are often still specified
prescriptively in terms of consistence as measured by the
slump of concrete. This approach may be suitable for
standard concrete mixes of moderate grade strength but is not
recommended for special concretes where higher performance
is required, especially if unrealistically low slump levels are
specified (e.g . slumps of 100 mm or less for higher strength
concrete). Some concrete specifications allow the consistence
level to be nominated by the contractor, which is a more
flexible and practical approach.
Prescriptive specifications for fresh concrete properties
become problematic with special concrete mixes that are often
significantly less workable at normal consistence levels such as
a slump of 100 mm. This is because many of these concretes
contain high powder contents and chemical admixtures that
significantly increase viscosity of the material. The result can be
that site limitations on slump ultimately reduce the hardened
properties of concrete due to inadequate compaction, poor
productivity and other early-age problems.
Concrete mixes require some optimisation to improve
economics and ensure appropriate performance while retaining
sufficient robustness for practical use on site. This process is
currently done by experienced concrete technologists who have
a good feel for their materials and concrete mix designs. A
complementary technique is presented using rheological testing
of fresh concrete, which provides scientific backing and allows
better understanding of the issues to other engineers. This paper
seeks to present rheology in a practical manner that reinforces
some of the issues commonly encountered by readymix
(premix) concrete suppliers.
2.0 RHEOLOGY AND RHEOCHARTS
Rheology is the science of deformation and flow of material
and is used in concrete technology to characterise the fresh
properties of the material. Two properties were measured using
a Contec BML4 viscometer that effectively shears concrete by
applying a stepped rotational cycle:
•
Yield shear stress (T0) is the boundary between liquid and
solid behaviour and effectively provides an estimate of the
plastic stiffness of concrete (e.g . self-compacting concrete
would be close to zero while kerb concrete would be over
2000 Pa).
•
Plastic viscosity (μ) represents the change in resistance of the
concrete at increasing shear and represents the stickiness of
fresh concrete (e.g. normal concrete viscosity is between 40-
80 Pa.s while some high strength geopolymer concretes may
be over 200 Pa.s).
The application of rheology to concrete technology is not new
and the science is well developed internationally.1 Probably the
most useful application of rheology is in rheocharts, which
help characterise the optimum range for different concrete
applications. A typical overview for concrete is shown in
Figure 1 based on the author’s experience and findings from
researchers.2
Figure 1: Rheochart showing typical ranges of concrete after Wallevik.2
Plastic viscosity (Pa.s)
Yieldshearstress(Pa)
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